The following disclosure is related to electrical circuits and signal processing.
Dimming for lamp is often used with switching power supply. Traditional dimming is realized by two ways:
1. Output current through LED lamp is a train of pulsed square waveform with high frequency; the duty cycle of the pulse current is adjusted to dim the LED lamp. This bring flicker at high frequency.
FIG. 1 shows a DC to DC or AC to DC converter used as power supply for the LED lamp with conventional pulsed current dimming way.
FIG. 2 shows the current waveform in the LED lamp of FIG. 1 with conventional pulsed current dimming way.
Output lamp brightness is proportional to output power of the DC to DC or AC to DC converter. Output power P=average(Vo*Io), Vo is constant, Io=IfI (IfI is current at full load) during on time Ton, Io=0 during off time Toff. So P=Vo*average(Io); P=Vo*IfI*(Ton/Ts)=Vo*IfI*D. (D is duty cycle and D=Ton/Ts). Output lamp brightness is proportional to duty cycle because Vo and IfI are constant. So if we want to dim lamp at 10% brightness, we only need to set D=10%. Pupil usually opens wide at darkness and shrinks at brightness. Because the frequency is high, pupil cannot follow the change of light. So strong light stimulate retina during Ton with high frequency, long time the stimulation will hurt eyes and eyesight.
2. The output current is like rectified sinusoidal waveform in which partial of the waveform is 0; the conduction angle of input voltage for lamp is adjusted to dim the LED lamp. This way brings flicker at low frequency around 100 Hz or 120 Hz.
FIG. 3 shows an AC input power supply with a phase dimmer for LED lamp. A phase dimmer adjust conduction angle of input AC voltage. The phase dimmer is connected between AC input voltage source and bridge rectifier. And the voltage after rectified is directly applied to the lamp.
FIG. 4 shows voltage across LED and current through LED during dimming in FIG. 3. Output current is partial sinusoidal waveform. And output lamp is on when the voltage is above threshold voltage but output lamp is off when the voltage is less than threshold voltage. The lamp brightness dimming is just average of output power in one period. The average value of output power is higher, the lamp is brighter; the average value of output power is lower, the lamp is darker. Lamp brightness varies with low frequency (100 Hz or 120 Hz), the pupil open wide at darkness and shrink at brightness. Then the muscles to control the pupil keep shrinking and relaxing 100 or 120 times per second. So the eyes are very tired and easy to get myopia.